For 3GPP Long Term Evolution (LTE) uplink Release 10, a contention based (CB) transmission scheme has been proposed to improve the latency of the system. The contention based scheme basically means broadcasting a grant to the cell to which any uplink time aligned User Equipment (UE), that has data to send may respond with a data transmission. Since many user equipments can use the grant, it is difficult to use the Hybrid Automatic Resend reQuest (HARQ) protocol to correct transmission errors, since typically errors are caused by interference from other user equipment sending data at the same time.
In order to improve performance of transmission in both the downlink and uplink direction, LTE in non-contention based mode uses Hybrid Automatic Repeat Request (HARQ). The basic idea of HARQ, for uplink transmission, is that after receiving data in an uplink subframe the eNB, or base station as it also may be referred to, attempts to decode it and then reports to the user equipment whether the decoding was successful by sending an acknowledgement (ACK) or unsuccessful by sending a negative acknowledgement (NACK). In the latter case of an unsuccessful decoding attempt, the user equipment thus receives a NACK in a later downlink subframe, and can retransmit the data that was not successfully received at the eNB.
However, in the case of a decoding error in a contention based transmission scheme, the eNB will not be able to determine the identity of the transmitting user equipment, and will therefore not be able to correctly combine (re-)transmissions from different user equipments transmitting of the same grant. Thus, the HARQ operation is disabled in the contention based transmission scheme.
Yet, having a retransmission scheme would significantly improve the performance in the cases where the transmissions do collide or are otherwise not successful.
In addition, a missed assignment will in general result in block errors that need to be corrected by higher-layer protocols, which in turn has a negative impact on performance in terms of throughput and latency. Also, increasing the delay may cause undesirable interactions with Transmission Control Protocol (TCP) based applications.